Stress recovery ratio

Based on the thermomechancial cycle, the stress recovery ratio and recovered stress can be determined. The constrained stress recovery ratio (which is defined as the ratio of the recovered stress over the programming stress in Eigure 3.29) and the recovered stress for each combination of test parameters are summarized in Table 3.3. 

For the programming temperature of 71 °C, it is seen that as the pre-strain level increases, the confined stress recovery ratio decreases. This is because as the pre-strain level increases, the... 

Table 3.3 Stress recovery ratio and recovered stress ...

Stress recovery ratio (%) Recovered stress (MPa) Stress recovery ratio (%) Recovered stress (MPa)... 

The recovered stress depends on both the stress recovery ratio and the programming stress. For the programming stress, the higher the pre-strain level, the higher is the programming... 

In order to calculate the stress fixity ratio and stress recovery ratio, we also need to define the pre-stress and the fixed temporary stress. Taking the stress-strain behavior during colddrawing programming as an example, both the stress and strain can be divided into several components, as shown in Figure 5.28. Similar to the definition of pre-strain, we can define the pre-stress as the peak applied stress, that is, stress at the end of the loading step (step 1), <7p. The challenge is how to define the fixed stress. 

In a manner similar to the shape fixity and the shape recovery ratio, we can define the stress fixity and the stress recovery ratio. From Figure 5.28, the temporarily fixed stress can be obtained by subtracting the springback stress from the total stress (peak stress). Once the... 

We can also define the stress recovery ratio Sr as the recovered stress over the fixed stress ... 

Obviously, the stress recovery ratio is much lower than the strain recovery ratio. The difference is even bigger for cold-drawing programming. In other words, the SMPF has a good memory of strain or shape, but a poor memory of stress or load. Actually, this is trae not only for SMPFs but also for almost all shape memory polymers. On the one hand, this shows the limitations of SMPs in certain applications, such as those that need a large recovery force. On the other hand, this also provides a unique opportunity for researchers to develop SMPs with a higher recovery stress and a higher stress recovery ratio. 

In general, SMPFs have a good memory of strain or shape, but a poor memory of stress or load. It is expected that, if other properties such as conductivity, color, density, etc., were used to evaluate the memory capability of SMPs, different recovery ratios would be obtained. Therefore, the memory capability depends on the parameter or criterion used. Although the current dominant criterion used to evaluate memory capability is the shape or strain recovery ratio, it is expected that other indicators such as the stress recovery ratio would be more and more used because SMPs will find new applications in various areas. Most importantly, we need to better understand the underlying physics for each parameter because different parameters may not be related linearly or proportionally. It is expected that this opens... 

The crack-closing ability of the composites with various CP contents can also be understood from the point of view of the stress recovery ratio. The stabilized programming stress (SPS) is the stress at point C. The stabilized confined recovery stress (SCRS) is the stress at point F. If the stress recovery ratio is defined as the ratio of the SCRS at point F to the SPS at point C, the stress recovery ratios of the composites are 66%, 60%, and 44% when the CP contents are 3%, 6%, and 9%, respectively. Therefore, we again see that the crack closing capability reduces as the CP content increases. However, this does not mean that the less of the CP content the better of the healing efficiency. The reason is that the CTH needs two steps, close and then heal. The above conclusion is based on the first step. For the healing step, it is obvious that the CP content must at a certain level. Otherwise there is no sufficient CP to fill in the narrowed crack and diffuse into the fractured PSMP matrix, that is, no healing. Therefore, an optimized CP content exists in practice. 

It is interesting to note that, while the CP content has a significant effect on the stress recovery, its effect on shape recovery is comparatively small. The reason is that shape recovery is basically a global or macroscopic behavior, while stress recovery is dependent more on the microstmeture and internal parameters. For instance, the recovery stress depends on the stiffness at the recovery temperature, while the recovery strain does not. Therefore, the shape recovery ratio and the stress recovery ratio usually do not have the same value and the stress recovery ratio is usually lower than the strain recovery ratio. In this sense, the stress recovery ratio is a more rigorous indicator of shape memory functionality. 

Again, here we notice the considerable difference between the stress recovery ratio and the strain (shape) recovery ratio. The reason is that shape recovery is a macroscopic measurement... 

Three-point flexural tests provide values for the modulus of elasticity in bending, flexural stress, flexural strain, and the flexural stress-strain response of the tested material [63, 64], i.e., the determination of strain recovery and stress recovery ratio. 

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